Geothermal ground source/water source heat exchange systems typically use fluid-filled closed loops of tubing buried in the ground, or submerged in a body of water, so as to either absorb heat from, or to reject heat into, the naturally occurring geothermal mass and/or water surrounding the buried or submerged fluid transport tubing. The tubing loop is extended to the surface for circulating the naturally warmed or cooled fluid to an interior air heat exchanger.
Older geothermal heating/cooling systems typically include three heat exchange steps. In a first heat exchange step, these systems circulate a fluid, which is typically water or water mixed with anti-freeze, in plastic (typically polyethylene) underground geothermal tubing, which transfers geothermal heat to or from the ground. In a second heat exchange step, a refrigerant heat pump system transfers heat to or from the water. Finally, in a third heat exchange step, an interior air handler (which may comprise finned tubing and a fan) transfers heat to or from a refrigerant to heat or cool interior air space.
More recent geothermal heating/cooling systems are called “direct exchange” or “DX” systems, which typically have only two heat exchange steps. In DX systems, the refrigerant fluid transport lines are placed directly in the sub-surface ground and/or water. The transport lines are typically made of copper tubing and the refrigerant fluid is typically R-22, R-410a, or the like, so that geothermal heat is transferred to or from the sub-surface elements in a first heat exchange step. A second heat exchange step transfers heat to or from an interior air space, typically by way of an interior air handler. Consequently, DX systems are generally more efficient than water-source systems because fewer heat exchange steps are required and because no water pump energy expenditure is necessary. Further, since copper is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the copper tubing of a DX system generally has a greater temperature differential with the surrounding sub-surface elements than the water circulating within the plastic tubing of a water-source system, generally, less excavation and drilling is required, and installation costs are lower.
While the earlier DX heat exchange designs performed sufficiently for their intended use, various improvements have been made to enhance overall system operational efficiencies. Several such design improvements, particularly in direct expansion/direct exchange geothermal heat pump systems, are taught in U.S. Pat. No. 5,623,986 to Wiggs; in U.S. Pat. No. 5,816,314 to Wiggs, et al.; in U.S. Pat. No. 5,946,928 to Wiggs; and in U.S. Pat. No. 6,615,601 B1 to Wiggs, the disclosures of which are incorporated herein by reference. The systems disclosed in these patents encompass both horizontally and vertically oriented sub-surface geothermal heat exchanger. While these previous systems have improved the performance of DX systems, additional improvements in DX system operational efficiencies would be advantageous.